Celebrate International Sloth Day

Happy International Sloth Day! To find out more about these adorable critters, we talked to Bryson Voirin, one of the first scientists to study sleep in animals in the wild — who started researching sloths because he was good at climbing trees.

Getting to Know the Sloth

Wild sloths are found only in Central and South America. There are two kinds of sloths — two-toed and three-toed — but surprisingly, these two types are not very closely related. They are similar because they both evolved to fit the same upside-down-hanging, leaf-eating niche. "They shared a common ancestor, which was a ground sloth, about 40 million years ago," says Voirin. "They evolved this arboreal lifestyle separately. It's convergent evolution at its finest."

The two kinds of sloths are different in a number of ways. Two-toed sloths are nocturnal, while the three-toed variety are what's called polyphasic — they can be active any time of day or night. "They don't have a specific sleep period," says Voirin. "They sleep for an hour here, an hour there." Three-toed sloths have eight to nine neck bones, while two-toed have five to seven. What makes this really strange is that most vertebrates have the same number of neck bones no matter how long their necks are; for example, both a human and a giraffe have seven.

Finally, their diets are different. Two-toed sloths eat fruit in addition to leaves, but three-toed sloths are much more specialized. They only eat leaves and buds, and what's more, they like to stick to Mom's home cooking: "They will get their specific diet from their mother — each individual will have a favorite tree, usually the favorite tree of their mother," Voirin tells us.

If you've seen a sloth at the zoo, it's almost certainly been two-toed. The three-toed variety doesn't do well in captivity, especially outside of the tropics. No one is sure why, although it probably has something to do with their diet.

A New Species of Sloth

Within the two types, there are six living sloth species: two kinds of two-toed and four kinds of three-toed, including the pygmy and the maned. The pygmy sloth, which was only recognized as a separate species in 2001, is critically endangered; found only on a single island in Panama, there are probably fewer than a thousand.

Voirin says that they're the easiest sloth to do research on. "It's much easier to catch them — you can just climb the mangrove tree and pick them up," he says. "Other three-toed will run away from you, but pygmy sloths will just sit there."

The difference is pygmy sloths have no experience of predators, which are absent from their island home. Other sloths run away because they know they're tasty — they're eaten by other animals including eagles and various cats. "Sixty percent of the diet of ocelots has been found to be sloths," he says.